Newly published research analyzed more than 100,000 seawater samples worldwide and found the oceans are absorbing about 31 percent of human-caused carbon emissions. It’s “a huge service the oceans are doing,” says a co-author, in Seattle.

Richard Feely has spent years of his career at sea, casting packages of plastic tubes into its void, and pulling up seawater from its depths while exploring how carbon emissions are changing the world’s oceans.

Feely, a senior scientist at the National Oceanic and Atmospheric Administration (NOAA) Pacific Marine Environmental Laboratory in Seattle, has sampled Antarctic waters, passed through Tahiti and ridden out “huge” storms aboard ocean-class research vessels.

Most days on these scientific cruises, which last more than a month, there’s no other boat in sight.

“You see an occasional whale or some birds,” he said. “It’s a big ocean.”

It’s hard, tedious work.

But the data collected by Feely and other scientists throughout the world are now telling us just how much carbon humans have put into the ocean and what that might mean for our future.

Research published Thursday in the peer-reviewed journal Science analyzed more than 100,000 seawater samples worldwide collected from 1994 to 2007 and taken from nearly every corner and depth of ocean.

The analysis found the oceans are absorbing about 31 percent of the carbon humans are spewing into the world. For context, the weight of the carbon seeping into the ocean each year, on average for the period of study, is roughly equivalent to 2.6 billion Volkswagen Beetle cars, Feely said.

It’s “a huge service the oceans are doing,” said Feely, who is listed as a co-author on the study. “That significantly reduces global temperature.”

But that temperature buffering comes at a cost. The ocean has continued to acidify, and changes in its chemistry are already affecting ecosystems in the Pacific Northwest. Adding carbon lowers seawater’s pH level, making it more corrosive. The research by Feely and others found the uptake of carbon in the world’s oceans has kept pace with worldwide CO2 emissions into the atmosphere.

“The amount of carbon in the ocean, that rate is increasing, because the amount of CO2 we’re releasing into the atmosphere is still increasing,” Feely said.

To scientists, the results of the Science study were less a revelation and more of an affirmation. Thousands of physical measurements painstakingly collected over 13 years in nearly every reach of the ocean matched what they already knew.

“The oceans have been taking up carbon dioxide recently in exactly the way we thought they would,” said Curtis Deutsch, a University of Washington associate professor of oceanography, who was not involved in the research.

During 13 years of data collection, scientists took more than 50 cruises and collected more than 100,000 seawater samples, according to NOAA senior scientist Richard Feely. (NOAA)

“There’s nothing really surprising about the results, but they are super important in confirming that we really do understand the system and the way it operates.”

Scientists have long relied on climate models to forecast our warming future. In the ocean, the models consider factors like circulation, biological processes and the chemistry of carbon dioxide and water.

“Those models all turned out to be correct and that’s really an important fundamental step in the scientific process: that you verify predictions that are made based on basic principles of ocean circulation,” Deutsch said.

Those circulation patterns are incredibly important, and scientists are keenly watching for large-scale changes, he said.

The Atlantic Ocean, for example, is about half the size of the Pacific Ocean, but the research shows the Atlantic absorbs nearly as much carbon. That’s because ocean circulation patterns in the Atlantic churn waters down to greater depths than in the Pacific, which means the effects of human-caused carbon emissions can be detected more than a mile below the ocean surface.

If ocean circulation were to slow down, some scientists have hypothesized, the ocean would not be able to absorb as much carbon. That could intensify the rate of global warming because additional carbon would remain in the atmosphere and not be absorbed by the ocean.

The data analyzed in the Science paper hints at a slowdown in circulation, Deutsch said, but because the length of study is only 13 years, the cause is not clear, and it could be natural and benign.

“We think this is more in line with natural variability than long-term change at this point in time,” Feely said. “That remains to be seen.”

The Pacific Northwest could be particularly sensitive to increases in carbon in the ocean.

“Here in the Pacific Northwest, there are some somewhat unique factors that result in our experience of ocean acidification being more intense than in other places,” said Jan Newton, a UW oceanographer who co-directs the Washington Ocean Acidification Center.

Along the Washington coast in the summer, upwelling, an ocean pattern that surfaces deep water, brings up waters high in nutrients and CO2 and low in oxygen, Newton said. Meanwhile, in Puget Sound, tidal mixing brings up carbon-rich deep water.

So, humans are adding carbon to waters that are already naturally more corrosive.

“It can be noticeable,” Newton said. “We’re starting to see some of these effects more quickly than other places in the world.”

Ocean acidification already has hampered the shellfish industry. A more acidified ocean can prevent oysters and other shellfish from forming their shells.

UW research has shown that high-CO2 water can erode the ability of some salmon to process smells, critical for the fish to avoid predators.

More-corrosive waters have been shown to reduce the reproductive success of krill, Newton added.

“Some organisms are going to be resilient and adapt, but it’s an overall stress” to have additional carbon in the water, Newton said. “At some point, there becomes a limit and that’s the part that, to some extent, is impossible to know. And that’s the part we really need to be considering.”

Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, Santa Barbara, CA, United States

While the value of giant kelp (Macrocystis pyrifera) as a habitat-forming foundation species is well-understood, it is unclear how they impact the oxygen concentration and pH of the surrounding seawater, and further, how such a dynamic abiotic environment will affect eco-evolutionary dynamics in a context of global change. Here, we profiled the nearshore kelp forest environment in Southern California to understand changes in dissolved oxygen (DO) and pH with high spatiotemporal resolution. We then examined transgenerational effects using sea urchins (Strongylocentrotus purpuratus) as our study organism. Using enclosures on the benthos, we conditioned adult sea urchins in situ at two locations – one inside the kelp forest and one outside the kelp forest. After a 11-week conditioning period timed to coincide with gametogenesis in the adults, the urchins were collected, spawned, and cultures of their progeny were raised in the laboratory in order to assess their performance to simulated ocean acidification. In terms of the physical observations, we observed significant changes in DO and pH not only when comparing sites inside and outside of the kelp forest, but also between surface and benthic sensors at the same site. DO and pH at the benthos differed in mean, the amplitude of the diel signal, and in the profile of background noise of the signal. Ultimately, these results indicated that both DO and pH were more predictably variable inside of the kelp forest environment. On the biological side, we found that adult sea urchins inside the kelp forest produced more protein-rich eggs that developed into more pH-resilient embryos. Overall, this study in a temperate kelp forest ecosystem is one of the first studies to not only observe biological response to highly characterized environmental variability in situ, but also to observe such changes in a transgenerational context.

]]>Elevated CO2 alters behavior, growth, and lipid composition of Pacific cod larvaehttps://c-can.info/2019/02/26/elevated-co2-alters-behavior-growth-and-lipid-composition-of-pacific-cod-larvae/
Tue, 26 Feb 2019 18:21:58 +0000https://c-can.info/?p=2843High-latitude seas, which support a number of commercially important fisheries, are predicted to be most immediately impacted by ongoing ocean acidification (OA). Elevated CO2 levels have been shown to induce a range of impacts on the physiology and behavior of marine fish larvae. However, these responses have yet to be characterized for most fishery species, including Pacific cod (Gadus macrocephalus). Based on laboratory experiments, we present a multi-faceted analysis of the sensitivity of Pacific cod larvae to elevated CO2. Fish behavior in a horizontal light gradient was used to evaluate the sensitivity of behavioral phototaxis in 4–5 week old cod larvae. Fish at elevated CO2 levels (∼1500 and 2250 μatm) exhibited a stronger phototaxis (moved more quickly to regions of higher light levels) than fish at ambient CO2 levels (∼600 μatm). In an independent experiment, we examined the effects of elevated CO2 levels on growth of larval Pacific cod over the first 5 weeks of life under two different feeding treatments. Fish exposed to elevated CO2 levels (∼1700 μatm) were smaller and had lower lipid levels at 2 weeks of age than fish at low (ambient) CO2 levels (∼500 μatm). However, by 5 weeks of age, this effect had reversed: fish reared at elevated CO2 levels were slightly (but not significantly) larger and had higher total lipid levels and storage lipids than fish reared at low CO2. Fatty acid composition differed significantly between fish reared at high and low CO2 levels (p < 0.01) after 2 weeks of feeding, but this effect diminished by week 5. Effects of CO2 on FA composition of the larvae differed between the two diets, an effect possibly related more to dietary equilibrium and differential lipid class storage than a fundamental effect of CO2 on fatty acid metabolism. These experiments point to a stage-specific sensitivity of Pacific cod to the effects of OA. Further understanding of these effects will be required to predict the impacts on production of Pacific cod fisheries.

Oregon State University researchers have documented tanner crabs feeding at a methane seep site off British Columbia. Tanner crabs are also known as ‘snow crabs’ and sold as food. It is the first time a commercially harvested species has been known to feed at methane sites.The methane shouldn’t cause any health concerns and, in fact, it may provide an alternative energy source for seafloor-dwelling marine species. [Oregon State University]

Climate change will result in less ocean-borne food falling into the deep sea, scientists say. But that likely won’t be a problem for tanner crabs, according to a recent discovery by Oregon State University researchers.

The long-legged orange crabs — one of three species that crabbers harvest and sell as snow crabs — vigorously feed at methane seeps, where the gas bubbles up from the ocean floor.

“The thinking used to be that the marine food web relied almost solely on phytoplankton dropping down through the water column and fertilizing the depths,” OSU Marine Ecologist Andrew Thurber said in a statement. “Now we know that this viewpoint isn’t complete and there may be many more facets to it.”

Thurber co-authored a study that the journal Frontiers in Marine Science just published. The study details how scientists found tanner crabs in eating frenzies around a methane seep in the floor of the Pacific Ocean off British Columbia. It is one the first times that a commercially harvested seafood has been found to rely on methane seeps.

Methane seeps appear to be serving up food to seafloor-dwelling species, such as tanner crabs. This would be a hedge against climate change because nearly all models predict less food will drop into the deep sea in coming years.

“Tanner crabs likely are not the only species to get energy from methane seeps, which really haven’t been studied all that much,” Thurber said. “We used to think there were, maybe, five of them off the Pacific Northwest coast and now research is showing that there are at least 1,500 seep sites — and probably a lot more. … They are all over the world, so the idea that they may provide an energy source is quite intriguing.”

Researchers first noticed tanner crabs bunching up around methane seeps in 2012 off the British Columbia coast. The crabs sifted through sediment at the bubbling seeps. Mats of bacteria form around the seeps and the crabs munch on those.

Underwater video shows methane building up below tanner crabs hanging out at seeps and eventually flipping them. The entertaining video drew researchers to wonder why the crabs were gathered around the seeps in the first place.

OSU teamed up with scientists from the University of Victoria in Canada. The National Science Foundation in the U.S. provided support for the study.

Off the Oregon Coast, Pacific sole and black cod have been seen near methane seeps. Like the crabs, the fish are harvested.

But seafood lovers need not worry about what their food is eating. Researchers say methane seeps create nontoxic environments.

Sarah Seabook, the lead author of the study and a Ph.D. candidate at OSU, said scientists examined the guts and tissues of tanner crabs to confirm they were feeding around methane seeps.

″… We can apply these new techniques to other species and find out if the use of methane seeps as a food source is more widespread than just tanner crabs,” she said in a statement.

]]>Ocean acidification could mean bad news for Dungeness crabhttps://c-can.info/2019/02/20/ocean-acidification-could-mean-bad-news-for-dungeness-crab/
Wed, 20 Feb 2019 19:27:42 +0000https://c-can.info/?p=2838As local fishermen continue to ply their trade in Del Norte County waters, scientists are studying the effects of climate change on a myriad of species including Dungeness crab.

In an action plan published October 2018, the State of California outlines priorities for research into ocean acidification caused by global carbon dioxide emissions by the burning of fossil fuels, including prevention and adaptation.

According to Jessica Williams, project scientist for the Ocean Science Trust, which has been working with the California Ocean Protection Council, though the research into the impacts of ocean acidification is new, there will be a lot of impacts to several different species. This includes Dungeness crab as well as the algal blooms that contribute to fisheries closures due to domoic acid, Williams said.

“Scientists are still trying to figure out what those impacts will be,” she said. The Ocean Science Trust released an infographic recently focusing on the impacts of ocean acidification. “This infographic (is) demonstrating what we currently know about the direct impacts and there’s active research, not only for direct impact on species and what that means, but indirect impacts we might not see.”

One scientist, Paul McElhany, at NOAA’s Northwest Fisheries Science Center in Seattle, has been part of an experiment trying to understand the sensitivity of Dungeness crab to the high carbon dioxide levels that contribute to ocean acidification.

He said he and his colleagues focus on the larval stage, rearing crab under controlled carbon dioxide conditions that simulate the current environment as well as a projected future environment with higher carbon dioxide conditions.

“What we’ve seen so far is under high CO2 conditions, larvae have a lower survival and slower development rate,” McElhany said.

So far, it’s been a challenge to extrapolate what that might mean for the overall Dungeness crab population, according to McElhany. He and his fellow researchers are also running a number of other experiments expanding on those initial results, including trying to determine how crab do under multiple stressors such as changes in temperature and changes in oxygen concentrations. They’ve also studied how crab at later developmental stages respond to higher carbon dioxide concentrations, McElhany said.

Crab typically mate in the spring, McElhany said. The female then extrudes her eggs, deposits them onto her abdomen in the fall, nestles down in sediment over the winter and the eggs hatch in the spring. Hatchlings, called zoea, don’t look at all like a crab, McElhany said, and live in the water column. The transition from this stage to maturity can take two or more years, he said.

“In the first set of experiments, we took females that had eggs already attached to them into the lab,” McElhany said. “When those eggs hatched, we reared (them) in the zoea stage in water with controlled CO2 conditions, current CO2 levels and future CO2 levels. What we saw in those experiments is the zoea under higher CO2 conditions, they had lower survival (rates). Not as many lived as compared to current day CO2 and they also developed slower.”

At later juvenile stages, McElhany said it appears that crab are more resilient to higher CO2 conditions than they are at the zoea stage, though those results haven’t been published yet. However, he noted that slower development can cause them to delay reaching necessary milestones as the season progresses.

“They need to get to a certain stage before winter sets in,” McElhany said. “The slower they grow, the longer they’re vulnerable to other predators. The quicker they get big, the harder it is for other things to eat them. We haven’t shown those impacts, but that’s a concern with slower developmental rate.”

According to McElhany, the amount of carbon dioxide in the water can vary. Concentrations in the Puget Sound area on average are higher than on the Washington coast. He said it has largely to do with ocean circulation patterns and the input of fresh water, noting that freshwater has a lower pH than sea water.

However, McElhany said, because of the upwelling pattern in the Pacific Ocean, there are some areas on the West Coast with high concentrations of carbon dioxide. That water also tends to be lower in oxygen, but higher in nutrients, he said.

Ocean acidification can have a potential impact on the Dungeness crab fishery, especially because of its potential effects on domoic acid. Though the commercial season in Crescent City normally starts on Dec. 1, unsafe levels of the toxic algae delayed the season until Jan. 25.

]]>‘Underwater forecast’ predicts temperature, acidity and more in Puget Soundhttps://c-can.info/2019/02/11/underwater-forecast-predicts-temperature-acidity-and-more-in-puget-sound/
Mon, 11 Feb 2019 21:31:13 +0000https://c-can.info/?p=2834Most of us rely on the weather forecast to choose our outfit or make outdoor plans for the weekend. But conditions underwater can also be useful to know in advance, especially if you’re an oyster farmer, a fisher or even a recreational diver.

A new University of Washington computer model can predict conditions in Puget Sound and off the coast of Washington three days into the future. LiveOcean, completed this past summer, uses marine currents, river discharges and weather above the water to create the forecasts.

“It’s like a weather forecast of the ocean in our region,” said lead developer Parker MacCready, a UW professor of oceanography. The project is the culmination of about 15 years of work. “It started off small, modeling parts of Puget Sound, and went to modeling the Columbia River and the coastal ocean nearby, to modeling the whole region. We’re making the model bigger and more realistic all the time.”

Unlike existing marine forecasts that tell boaters the wind and waves out on the water, this model drops below the water’s surface to predict water temperature, salinity, oxygen, nitrogen, pH, chlorophyll — a sign of biological productivity — and aragonite saturation, the most important factor in shell formation, from the surface down to the seafloor.

The simulations are updated daily on the UW’s Hyak supercomputer with a resolution of 500 meters (about a third of a mile) throughout Puget Sound, and slightly more for the outer coast, from southern Oregon to near the tip of Vancouver Island. The model incorporates 45 river flows, uses a UW weather forecast for wind, rain and sunlight, and compares its predictions against dozens of marine testing sites.

LiveOcean was originally developed to predict the impacts of more acidic seawater on the local shellfish industry, and has support from the state-funded Washington Ocean Acidification Center as a tool for local shellfish growers. This will be the first spring that the tool is available for their use.
“If growers buy seed from a hatchery, when’s a good time to put those out in the water?” MacCready said. “Is there predicted to be a very corrosive ocean acidification event? If so, they should hold off until the water becomes less acidified.”

The National Oceanic and Atmospheric Administration also funds the project. It uses the forecast in combination with human analysis to produce the joint UW-NOAA bulletin on harmful algal bloom forecasts, or “red tides,” that it shares with coastal managers.

The Puget Sound forecasts have other applications. Elizabeth Brasseale, a UW graduate student in oceanography, has used LiveOcean to predict where invasive green crab larvae might travel next, enabling Washington Sea Grant to pinpoint its green crab eradication efforts. The model can predict the three-day drift path for any object — spilled oil, wastewater overflow, trash or even an old-fashioned message in a bottle — released from a given point in Puget Sound.

The LiveOcean forecasts are now available on the UW-based Northwest Association of Networked Ocean Observing Systems website. To access the forecasts, click “Layers” at the top left, find “Models” and then scroll down to “LiveOcean” to view maps for temperature, salinity, oxygen, nitrogen, phytoplankton, pH as well as aragonite saturation. (Click the scale bar to make it bigger.)

LiveOcean is among a handful of seawater forecasts being developed for the Pacific Northwest. The SeaCast app, from Oregon State University, covers Oregon and Washington coasts. The SalishSeaCast from the University of British Columbia focuses on the Salish Sea, and the Salish Sea Model from the Pacific Northwest National Laboratory simulates the region’s water but does not issue forecasts.

MacCready compares the situation with global climate models, where models with different specialties give a better overall understanding of the system.
While the daily LiveOcean forecast is useful for making decisions today, the tool also has accumulated several years of historical simulations that allow people to analyze past events, like the unusually warm conditions off the Pacific Northwest coast that peaked in 2015.

“We know that our model is able to reproduce ‘the blob,’ and that it shows up really nicely,” MacCready said. “This new version will allow a much better exploration of what that event looked like inside the Salish Sea.”

LiveOcean builds on decades of experience with Puget Sound’s complex geography and intricate coastlines. In addition to helping managers, it’s intended to act as a teaching tool. MacCready has created documents on how tides work in Puget Sound, the long-term warming trend in Puget Sound and has written an accompanying primer on where Puget Sound’s water comes from.

“The big thing I try to explain to people is that we have this persistent current below the surface dragging deep, saline water into the Salish Sea, where it mixes with the freshwater and then flows out,” MacCready said. “That flow is 20 times bigger than all our rivers combined, and it brings in 95 percent of our nutrients. It’s really the biggest river in Puget Sound, but it’s actually coming uphill, from the deep ocean.”

As spring arrives in Puget Sound, the rains will let up, snow will melt and the rivers will begin to rise. Winds along the coast will soon reverse direction, which draws more nutrient-rich flow from the deep ocean. And residents of the Sound will be getting out on the water for activities of all kinds.
“Now that this makes daily forecasts and performs pretty well, I think it could be used for a lot more applications,” MacCready said. “I’d be delighted to hear from people with ideas.”

A summary of the latest research on ocean acidification (OA) impacts to important species and ecosystems in California, from crab to squid, rockfish to urchins. This tool provides a tangible illustration of our current knowledge to support decision-makers in prioritizing efforts and resources to address OA impacts.

Ocean Science Trust, working closely with scientists at UC Davis Bodega Marine Lab, the Ocean Protection Council (OPC) and other partners, undertook this synthesis to help identify data gaps and prioritize where to allocate resources to further increase understanding of OA impacts to California fishery resources.

Ocean acidification is a complex issue that has the potential to alter marine food webs and ecosystems in California, with direct and indirect impacts to valuable marine fisheries and the aquaculture industry. Currently, state agencies working to understand the risks OA poses to coastal species, ecosystems, and human communities – an essential step to helping those at risk prepare for what’s at stake as coastal oceans continue to acidify.

VISUALIZING IMPACTS OF OA TO LIVING MARINE RESOURCES IN CALIFORNIA

As a first step towards illuminating potential natural resource management solutions, Ocean Science Trust worked closely with scientists at UC Davis Bodega Marine Lab, the Ocean Protection Council and other partners to demonstrate the potential impacts of OA on important species and ecosystems in California. We undertook a synthesis of current scientific understanding and developed communications material for use by resources managers. The species included in the synthesis represent a diverse subset of species considered as ocean climate indicators, commercially, recreationally, and/or ecologically important. This list was selected by the project team and vetted and augmented by OPC, CDFW, and aquaculture representatives.

WORKSHOP: DEFINING OCEAN ACIDIFICATION HOTSPOTS IN CALIFORNIA

Building on this assessment, Ocean Science Trust hosted a workshop in November 2018, to help managers and decision-makers incorporate OA impacts information into relevant management decisions, prioritize efforts to address these impacts, and determine where to allocate resources to further increase understanding. This workshop brought together managers, policy makers, and scientists to better understand the concept of OA hotspots, ensure it is usable by state decision-makers, and identify key gaps in data and information that inhibit action.

Findings from this work may also:

Help identify research and data gaps to understanding OA impacts to California’s fishery resources

Inform species selection for a modeling exercise to identify species vulnerability thresholds

Provide the groundwork for a quantitative OA or climate vulnerability assessment for California or the West Coast

]]>Pacific geoduck (Panopea generosa) resilience to natural pH variationhttps://c-can.info/2019/01/13/pacific-geoduck-panopea-generosa-resilience-to-natural-ph-variation/
Mon, 14 Jan 2019 04:07:31 +0000https://c-can.info/?p=2819Pacific geoduck aquaculture is a growing industry, however little is known about how geoduck respond to varying environmental conditions, or how production might be impacted by low pH associated with ocean acidification. Ocean acidification research is increasingly incorporating multiple environmental drivers and natural pH variability into biological response studies for more complete understanding of the effects of projected ocean conditions. In this study, eelgrass habitats and environmental heterogeneity across four estuarine bays were leveraged to examine low pH effects on geoduck under different natural regimes, using proteomics to assess physiology. Juvenile geoduck were deployed in eelgrass and adjacent unvegetated habitats for 30 days while pH, temperature, dissolved oxygen, and salinity were monitored. Across the four bays pH was lower in unvegetated habitats compared to eelgrass habitats, however this did not impact geoduck growth, survival, or proteomic expression patterns. However, across all sites temperature and dissolved oxygen corresponded to growth and protein expression patterns. Specifically, three protein abundance levels (trifunctional-enzyme β-subunit, puromycin-sensitive aminopeptidase, and heat shock protein 90-⍺) and shell 16 growth positively correlated with dissolved oxygen variability and inversely correlated with mean 17 temperature. These results demonstrate that geoduck are resilient to low pH in a natural setting, 18 and other abiotic factors (i.e. temperature, dissolved oxygen variability) may have a greater 19 influence on geoduck physiology. In addition this study contributes to the understanding of how 20 eelgrass patches influences water chemistry.

Understanding the interactive effects of multiple stressors on pelagic mollusks associated with global climate change is especially important in highly productive coastal ecosystems of the upwelling regime, such as the California Current System (CCS). Due to temporal overlap between a marine heatwave, an El Niño event, and springtime intensification of the upwelling, pteropods of the CCS were exposed to co-occurring increased temperature, low Ωar and pH, and deoxygenation. The variability in the natural gradients during NOAA’s WCOA 2016 cruise provided a unique opportunity for synoptic study of chemical and biological interactions. We investigated the effects of in situ multiple drivers and their interactions across cellular, physiological, and population levels. Oxidative stress biomarkers were used to assess pteropods’ cellular status and antioxidant defenses. Low aragonite saturation state (Ωar) is associated with significant activation of oxidative stress biomarkers, as indicated by increased levels of lipid peroxidation (LPX), but the antioxidative activity defense might be insufficient against cellular stress. Thermal stress in combination with low Ωar additively increases the level of LPX toxicity, while food availability can mediate the negative effect. On the physiological level, we found synergistic interaction between low Ωar and deoxygenation and thermal stress (Ωar:T, O2:T). On the population level, temperature was the main driver of abundance distribution, with low Ωar being a strong driver of secondary importance. The additive effects of thermal stress and low Ωar on abundance suggest a negative effect of El Niño at the population level. Our study clearly demonstrates Ωar and temperature are master variables in explaining biological responses, cautioning the use of a single parameter in the statistical analyses. High quantities of polyunsaturated fatty acids are susceptible to oxidative stress because of LPX, resulting in the loss of lipid reserves and structural damage to cell membranes, a potential mechanism explaining extreme pteropod sensitivity to low Ωar. Accumulation of oxidative damage requires metabolic compensation, implying energetic trade-offs under combined thermal and low Ωar and pH stress. Oxidative stress biomarkers can be used as early-warning signal of multiple stressors on the cellular level, thereby providing important new insights into factors that set limits to species’ tolerance to in situ multiple drivers.